Forceatlas2_layout: slow?

## Description

Hi! Really interested in using Datashader to deal with large scale data-vis. I've been trying to go through the Networks part of the user guide and I wanted to use my own data. I have a dataset of 1,184,684 nodes and 1,210,193 edges. I've reshaped my original data so that it is two separate `nodes` and `edges` DataFrames, with the edges df providing source and target of the relevant nodes indexes in the `nodes` df.

Circular layout worked fine and produced a result within a few seconds. However...

```force_directed = forceatlas2_layout(nodes, edges, id='id', source='source',target='target')```

...has been running for about an hour with the process taking about 280% CPU and is yet to complete. I understand the mechanics of the force atlas layout are more complex than circular but I wondered if this amount of processing time is to be expected, and/or if there is a way to speed it up.

Thanks for all your efforts on this package. It's a great project.

## Your environment

* MacOS Mojave 10.14.3

```Hardware Overview:

Model Name: Mac Pro
Model Identifier: MacPro6,1
Processor Name: Quad-Core Intel Xeon E5
Processor Speed: 3.7 GHz
Number of Processors: 1
Total Number of Cores: 4
L2 Cache (per Core): 256 KB
L3 Cache: 10 MB
Memory: 12 GB
```
Conda Info

``` active environment : community_mapper
active env location : /Users/James/anaconda3/envs/community_mapper
shell level : 2
user config file : /Users/James/.condarc
populated config files : /Users/James/.condarc
conda version : 4.6.8
conda-build version : 3.17.6
python version : 3.7.1.final.0
base environment : /Users/James/anaconda3 (writable)
channel URLs : https://conda.anaconda.org/conda-forge/osx-64
https://conda.anaconda.org/conda-forge/noarch
https://repo.anaconda.com/pkgs/main/osx-64
https://repo.anaconda.com/pkgs/main/noarch
https://repo.anaconda.com/pkgs/free/osx-64
https://repo.anaconda.com/pkgs/free/noarch
https://repo.anaconda.com/pkgs/r/osx-64
https://repo.anaconda.com/pkgs/r/noarch
package cache : /Users/James/anaconda3/pkgs
/Users/James/.conda/pkgs
envs directories : /Users/James/anaconda3/envs
/Users/James/.conda/envs
platform : osx-64
user-agent : conda/4.6.8 requests/2.21.0 CPython/3.7.1 Darwin/18.2.0 OSX/10.14.3
UID:GID : 501:20
netrc file : None
offline mode : False```

Conda list

```# packages in environment at /Users/James/anaconda3/envs/community_mapper:
#
# Name Version Build Channel
appnope 0.1.0 py36_1000 conda-forge
asn1crypto 0.24.0 py36_1003 conda-forge
attrs 19.1.0 py_0 conda-forge
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blas 1.0 mkl anaconda
bleach 3.1.0 py_0 conda-forge
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ca-certificates 2019.3.9 hecc5488_0 conda-forge
certifi 2019.3.9 py36_0 conda-forge
cffi 1.12.2 py36h2d6ddff_1 conda-forge
chardet 3.0.4 py36_1003 conda-forge
click 7.0 py_0 conda-forge
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colorcet 1.0.0 py_0 conda-forge
cryptography 2.6.1 py36hc2b1221_0 conda-forge
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cytoolz 0.9.0.1 py36h1de35cc_1001 conda-forge
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decorator 4.4.0 py_0 conda-forge
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distributed 1.26.0 py36_1 conda-forge
entrypoints 0.3 py36_1000 conda-forge
freetype 2.10.0 h24853df_0 conda-forge
heapdict 1.0.0 py36_1000 conda-forge
idna 2.8 py36_1000 conda-forge
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intel-openmp 2019.3 199 anaconda
ipykernel 5.1.0 py36h24bf2e0_1002 conda-forge
ipython 7.3.0 py36h24bf2e0_0 conda-forge
ipython_genutils 0.2.0 py_1 conda-forge
jedi 0.13.3 py36_0 conda-forge
jinja2 2.10 py_1 conda-forge
jpeg 9c h1de35cc_1001 conda-forge
jsonschema 3.0.1 py36_0 conda-forge
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jupyter_core 4.4.0 py_0 conda-forge
jupyterlab 0.35.4 py36_0 conda-forge
jupyterlab_server 0.2.0 py_0 conda-forge
kiwisolver 1.0.1 py36h04f5b5a_1002 conda-forge
libcxx 4.0.1 h579ed51_0
libcxxabi 4.0.1 hebd6815_0 conda-forge
libffi 3.2.1 h6de7cb9_1006 conda-forge
libgfortran 3.0.1 h93005f0_2 anaconda
libpng 1.6.36 ha441bb4_1000 conda-forge
libsodium 1.0.16 h1de35cc_1001 conda-forge
libtiff 4.0.10 h79f4b77_1001 conda-forge
llvmlite 0.26.0 py36h3fea490_1000 conda-forge
locket 0.2.0 py_2 conda-forge
markupsafe 1.1.1 py36h1de35cc_0 conda-forge
matplotlib-base 3.0.3 py36hf043ca5_0 conda-forge
mistune 0.8.4 py36h1de35cc_1000 conda-forge
mkl 2019.3 199 anaconda
mkl_fft 1.0.10 py36h5e564d8_0 anaconda
mkl_random 1.0.2 py36h27c97d8_0 anaconda
msgpack-python 0.6.1 py36h04f5b5a_0 conda-forge
multipledispatch 0.6.0 py_0 conda-forge
nbconvert 5.4.1 py_2 conda-forge
nbformat 4.4.0 py_1 conda-forge
ncurses 6.1 h0a44026_1002 conda-forge
networkx 2.2 py_1 conda-forge
notebook 5.7.6 py36_0 conda-forge
numba 0.41.0 py36h1702cab_1000 conda-forge
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numpy-base 1.16.2 py36h6575580_0 anaconda
oauthlib 3.0.1 py_0 conda-forge
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openssl 1.1.1b h01d97ff_2 conda-forge
packaging 19.0 py_0 conda-forge
pandas 0.24.2 py36h0a44026_0 anaconda
pandoc 2.7.1 0 conda-forge
pandocfilters 1.4.2 py_1 conda-forge
param 1.8.2 py_0 conda-forge
parso 0.3.4 py_0 conda-forge
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pickleshare 0.7.5 py36_1000 conda-forge
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pip 19.0.3 py36_0 conda-forge
prometheus_client 0.6.0 py_0 conda-forge
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psutil 5.6.1 py36h1de35cc_0 conda-forge
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pygments 2.3.1 py_0 conda-forge
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pymongo 3.7.2 py36h0a44026_0 conda-forge
pyopenssl 19.0.0 py36_0 conda-forge
pyparsing 2.3.1 py_0 conda-forge
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pysocks 1.6.8 py36_1002 conda-forge
python 3.6.7 h8dc6b48_1004 conda-forge
python-dateutil 2.8.0 py_0 conda-forge
pytz 2018.9 py36_0 anaconda
pywavelets 1.0.2 py36h917ab60_0 conda-forge
pyyaml 5.1 py36h1de35cc_0 conda-forge
pyzmq 18.0.1 py36h4cc6ddd_0 conda-forge
readline 7.0 hcfe32e1_1001 conda-forge
requests 2.21.0 py36_1000 conda-forge
requests-oauthlib 1.2.0 py_0 conda-forge
scikit-image 0.14.2 py36h0a44026_1 conda-forge
scipy 1.2.1 py36h1410ff5_0
send2trash 1.5.0 py_0 conda-forge
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sortedcontainers 2.1.0 py_0 conda-forge
sqlite 3.26.0 h1765d9f_1001 conda-forge
tblib 1.3.2 py_1 conda-forge
terminado 0.8.1 py36_1001 conda-forge
testpath 0.4.2 py_1001 conda-forge
tk 8.6.9 ha441bb4_1000 conda-forge
toolz 0.9.0 py_1 conda-forge
tornado 6.0.1 py36h1de35cc_0 conda-forge
traitlets 4.3.2 py36_1000 conda-forge
tweepy 3.6.0 py36_0 conda-forge
urllib3 1.24.1 py36_1000 conda-forge
wcwidth 0.1.7 py_1 conda-forge
webencodings 0.5.1 py_1 conda-forge
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xarray 0.12.0 py_0 conda-forge
xz 5.2.4 h1de35cc_1001 conda-forge
yaml 0.1.7 h1de35cc_1001 conda-forge
zeromq 4.2.5 h0a44026_1006 conda-forge
zict 0.1.4 py_0 conda-forge
zlib 1.2.11 h1de35cc_1004 conda-forge```